1. Field of the Invention
The present invention relates to a liquid crystal display device used for displays of office automation apparatuses, audio and video apparatuses, and the like, and a method for fabricating such a liquid crystal display device.
2. Description of the Related Art
A conventional transistor-type active matrix driven liquid crystal display device will be described with reference to FIGS. 6A and 6B, as the first prior art example. FIG. 6A is a perspective view of the conventional liquid crystal display device, and FIG. 6B is a sectional view taken along line 6B--6B of FIG. 6A.
Referring to FIG. 6B, the conventional liquid crystal display device includes a first substrate 601 (hereinbelow, referred to as a counter substrate) and a second substrate 602 (hereinbelow, referred to as an active matrix substrate) disposed to face each other with a predetermined gap therebetween. A liquid crystal layer 612 is provided between the substrates 601 and 602. Referring to FIG. 6A, the active matrix substrate 602 includes: a plurality of parallel source bus lines 609 and a plurality of parallel gate bus lines 610 which are formed on the surface of a glass substrate 620 facing the liquid crystal layer 612; and thin film transistors (TFTs) 611 disposed at the respective crossings of the source bus lines 609 and the gate bus lines 610. Pixel electrodes 608 are formed on the surface of the glass substrate 620 facing the liquid crystal layer 612 and connected to drain electrodes of the corresponding TFTs 611. A voltage is applied to each of the pixel electrodes 608 for controlling the orientation direction of liquid crystal molecules in the liquid crystal layer 612.
Referring to FIG. 6B, the counter substrate 601 includes: a plurality of colored portions 605 on the surface thereof facing the liquid crystal layer 612 at positions corresponding to the pixel electrodes 608 of the active matrix substrate 602; and a counter electrode 603 formed covering the colored portions 605. A black matrix (BM) layer 604 made of a light-blocking material is disposed to fill gaps between the adjacent colored portions 605.
Hereinbelow, a color filter layer as used herein will be described. For example, when pixels are arranged in a strip shape, stripe-shaped colored portions of red (R), green (G), and blue (B) are arranged cyclically in parallel. A color layer is composed of a plurality of stripe-shaped color portions of a single color or a plurality of colored portions of a single color corresponding to respective pixel electrodes. Such color layers constitute a color filter layer. The color filter layer as used herein does not include the BM layer formed between the adjacent colored portions. Herein, a region of the liquid crystal display device defined by each of the pixel electrodes is called a pixel region.
Referring back to FIGS. 6A and 6B, in the conventional liquid crystal display device, a scanning signal voltage is sequentially applied to the gate bus lines 610 so as to switch on the TFTs 611 connected to the respective gate bus lines 610, thereby allowing a specific display signal voltage to be written in each of the pixel electrodes 608 and held for a certain time period. The liquid crystal layer 612 interposed between the substrates 601 and 602 is driven with the potential difference between the voltage at each of the pixel electrodes 608 and a counter voltage applied to the counter electrode 603.
Two exemplary methods normally used for fabricating the counter substrate having a plurality of colored portions as described above will be described with reference to FIGS. 7A to 7E and 8A to 8E. FIGS. 7A and 8A are flowcharts showing normal fabrication steps for the counter substrate having colored portions. FIGS. 7B to 7D and 8B to 8D are plan views, together with corresponding sectional views, at the respective fabrication steps. FIGS. 7E and 8E are sectional views of the respective complete counter substrates.
Herein, a normal dry film fabrication method will be described with reference to FIGS. 7A to 7E. A resin film (dry film) having red (R) pigments dispersed therein is laminated to a glass substrate 720, followed by steps such as exposure to light, development, and baking, to form an R color layer composed of a plurality of stripe-shaped R colored portions 705.
A dry film having green (G) pigments dispersed therein is then laminated to substantially the entire top entire top surface of the resultant substrate to form a counter electrode 703. Thus, a counter substrate 701 is fabricated. As the BM layer, a metal film may also be used as shown in FIG. 8B as a metal BM film 804.
FIGS. 8A to 8E illustrate a fabrication process of the counter substrate by a spin coat method. Resin materials having color pigments dispersed therein are applied to substantially the entire top surface of a glass substrate 820 by spin coating, to form R, G, and B colored portions 805, 806, and 807, so as to fabricate a counter substrate 801.
Hereinbelow, a common transfer portion formed in a conventional liquid crystal display device such as described above will be described with reference to FIGS. 9A to 9C. The common transfer portion as used herein refers to a portion for securing an electrical connection between the counter substrate and the active matrix substrate to be used as a terminal formed on the active matrix substrate for applying a voltage to the counter electrode.
FIG. 9A is a plan view, together with a corresponding sectional view, of a conventional liquid crystal display device. FIG. 9B is a plan view, together with a corresponding sectional view, illustrating a common transfer portion of an active matrix substrate 902 of the conventional liquid crystal display device. In FIG. 9B, the reference numerals 909 and 910 denote source bus lines and gate bus lines, respectively. FIG. 9C is a plan view, together with a corresponding sectional view, illustrating a common transfer portion of a counter substrate 901 of the conventional liquid crystal display device. In FIG. 9C, the reference numeral 905 denotes colored portions.
Referring to FIG. 9B, the active matrix substrate 902 includes a common transfer electrode 917 formed between adjacent source driver connection blocks in a source terminal extension portion. The common transfer electrode 917 is connected to a common line at a position on the periphery of the display panel, and is electrically connected with a counter electrode 903 (FIG. 9C) of the counter substrate 901 via carbon paste 918. The number of such common transfer electrodes 917 formed for one display panel may be appropriately determined depending on the definition level of the panel, the size of the panel, the difference in resistance from the transparent counter electrode, and the like. For example, for a panel equivalent to Type 10 VGA, about four to eight common transfer electrodes are normally provided.
Referring to FIG. 9C, in the counter substrate 901, ITO is deposited using a mask so that an ITO portion 903 is formed on the area of the counter substrate 901 corresponding to the common transfer electrode 917 of the active matrix substrate 902, so that the ITO portion 903 comes into contact with the carbon paste 918 of the active matrix substrate 902. The resultant connection portion of the counter substrate 901 has a structure of a glass substrate 920/a BM layer 904/the ITO portion 903/the carbon paste 918 formed in this order. Alternatively, ITO may be formed over the entire surface of the counter substrate 901, instead of masking. In this case also, the same structure as described above is obtained.
The counter substrate 901 and the active matrix substrate 902 fabricated as described above are placed to face each other with a predetermined gap therebetween. While a sealer 919 is provided between the substrates 901 and 902 along the periphery thereof, a liquid crystal material is injected into the gap between the substrates 901 and 902 so as to be sealed to form a liquid crystal layer 912. The liquid crystal display device is thus fabricated. When a twisted nematic (TN) liquid crystal material is used for the liquid crystal layer, the gap between the substrates 901 and 902 is normally set at about 4 to 5 .mu.m. Such a gap is realized by dispersing dielectric beads having a diameter of about 4.5 to 7 .mu.m over the entire surface of either the counter substrate 901 or the active matrix substrate 902. Such dielectric beads are dispersed in an unspecific manner over the entire surface of the substrate including the portions above the pixel electrodes as long as no aggregation or the like is generated.
In the first prior art example described above, as shown in FIG. 6B, the source bus line 609 and the counter electrode 603 form a capacitance component therebetween with only the liquid crystal layer 612 existing therebetween. Therefore, if a capacitance coupling is formed between the source bus line 609 charged with a signal and the counter electrode 603, a signal delay may be generated on the source bus line 609, generating a difference in write voltage between the signal input terminal and the signal non-input terminal of the source bus line 609. This reduces the display quality of the liquid crystal display device.
Another problem is as follows. Each of the pixel electrodes 608 is influenced by an electric field from not only the portion of the counter electrode 603 located right above the pixel electrode 608, but the entire counter electrode 603. This influence of the electric field from the counter electrode 603 will be described with reference to FIG. 6B. Liquid crystal molecules located near a point B on the pixel electrode 608 are strongly influenced by electric fields from points D, E, F, and the like on the counter electrode 603 closer to the point B. They are also influenced by electric fields including slant components from the points E, E, F, and the like. This may disturb the orientation of the liquid crystal molecules.
As a result of the disturbance of the orientation of the liquid crystal molecules, transmitted light from a backlight incident on the region of the pixel electrode 608 is scattered at points of the periphery of the pixel electrode 608 such as the point B. This reduces the contrast of the liquid crystal display devices.
A liquid crystal display device for minimizing the influence of the slant components of the electric field to reduce the display defect is disclosed in Japanese Publication for Opposition No. 2520595. This liquid crystal display device, as the second prior art example, includes a plurality of stripe-shaped counter electrodes in place of the counter electrode described in the first prior art example.
In order to form strip-shaped counter electrodes, however, a photolithographic step and an etching step are required to pattern the film for the counter electrode. This increases the number of steps, reduces the yield, and thus increases the production cost. Moreover, when the stripe-shape counter electrodes are formed for a large-size and/or high-precision liquid crystal display device, the interconnection resistance of the counter electrodes increases, reducing the display quality.
As the third prior art example, Japanese Laid-Open Publication No. 5-249494 discloses a liquid crystal display device where step on a substrate surface formed around bus lines are angularly controlled for reducing the generation of reverse title domains and thus improving the display quality. Reverse tilt domains are generated due to a failure in the control of the orientation direction of liquid crystal molecules caused by a failure in the alignment processing during the step of forming an alignment film. Alternatively, a liquid crystal display device having superficial concave grooves formed between adjacent pixel electrode portions is disclosed in Japanese Laid-Open Publication No. 7-20497.
However, in the above-described structures of the active matrix substrate, although the generation of the reverse tilt domains is suppressed, the problem of the influence of slant components of the electric filed is not solved. As a result, it is not possible to completely inhibit the generation of the reverse tilt domains.
As the fourth prior art example, Japanese Laid-Open Publication No. 6-82795 and No. 8-328020 disclose the following liquid crystal display device. That is, in order to reduce the amount of beads scattered on the surface of the pixel electrode portions to improve the display quality, a potential difference is provided between bus line regions made of metal and the like and the other regions to allow beads to attach only to specific portions.
However, in order to fabricate a liquid crystal display device with the above structure, respective bus lines must be charged. It takes time to position terminals for charging under substantially an equal pressure. Moreover, for a high-precision liquid crystal display device, a uniform charging is difficult, requiring the provision of a specific structure or step for interconnecting. Fabricating such a liquid crystal display device increases the production cost.
In the liquid crystal display devices of the first to fourth prior art examples described above, the components for driving the pixel electrodes, such as the switching elements, the gate bus lines, and the source bus lines, are disposed on the second substrate. In order to electrically isolate functional films (e.g., conductive films and semiconductor films) for these components from one another, the components are arranged with predetermined spaces from one another on the same plane. In the regions corresponding to such spaces, it is not possible to apply a voltage to the liquid crystal layer to control the light blocking and transmission be the liquid crystal layer. The black matrix (BM) layer therefore needs to be disposed on the first substrate to block light from these regions. In such liquid crystal display devices, the source bus lines are arranged in the regions where the BM layer is formed. Since the metal film constituting the source bus lines also serves as a light-blocking layer, only a small portion of the regions covered with the BM layer is substantially blocked from light by only the BM layer made of a photosensitive resin material.
In the liquid crystal display devices of the first to fourth prior art examples, the gate bus lines and the source bus lines are arranged on the same substrate via an insulating film therebetween. This tends to cause a short circuit therebetween, thereby reducing the production yield.
In order to solve the above problem, a structure where the source bus lines are arranged on the first substrate while the switching elements and the gate bus lines are arranged on the second substrate (hereinbelow, such a structure is referred to as a counter source structure) is disclosed in the following literature:
(1) J. F. Clerc et al., "New Electronics Architectures for Liquid Crystal Displays Based on Thin Film Transistors", Japan Display '86 PA1 (2) K. Oki el al., "New Active Matrix Full Color Liquid Crystal Display", ITEJ Technical Report, vol. 11, No. 27, pp. 73-78 PA1 (3) K. Oki et al., Japanese Laid-Open Publication No. 62-133478, "Active Matrix Display Device".
A liquid crystal display device having the counter source structure will be described with reference to FIG. 10 as the fifth prior art example.
The liquid crystal display device having the counter source structure includes source bus lines 1009 formed on a first substrate, and gate bus lines 1010, reference lines 1021 for applying a reference potential to a liquid crystal layer, pixel electrodes 1008, and switching elements 1011 formed on a second substrate. The first substrate and the second substrate are disposed facing each other with a predetermined gap therebetween. The liquid crystal layer is formed between the substrates. In the liquid crystal display device having the counter source structure, since no crossing between the gate bus lines 1010 and the source bus lines 1009 are formed on the second substrate, a short circuit between a gate bus line and a source bus line is prevented. This increases the yield in the fabrication of the liquid crystal display device. Moreover, since no crossings between the gate bus lines 1010 and the source bus lines 1009 are formed on the second substrate, the gate bus lines and the source bus liens are less affected by capacitance coupling, eliminating a problem of signal delay.
However, the following problem arises when the counter source structure shown in FIG. 10 is applied to a color liquid crystal display device.
For color display, a color filter layer composed of color layers of different colors which selectively transmit light beams having specific wavelengths must be formed on the first substrate. In the case of the counter source structure, the source bus lines are formed on the first substrate on which the color filter layer is formed. This means that no source lines made of a metal film exist at positions on the second substrate corresponding to the BM layer as in the case of the liquid crystal display device shown in FIGS. 6A and 6B. This necessitates the formation of a BM layer made of a photosensitive resin material and the like to block light from the regions other than the colored portions.
When a BM layer is provided, however, steps may be formed on the surface of the counter substrate (first substrate). In such a case, the orientation of liquid crystal molecules in the liquid crystal layer is disturbed in the vicinity of the steps, reducing the display quality. Therefore, in order to maintain good display quality, the control of the thickness of the BM layer is critical.